Polar Localizing Class V Myosin Chitin Synthases Are Essential during Early Plant Infection in the Plant Pathogenic Fungus Ustilago maydis

Isabella Weber ¹, Daniela Aßmann ¹, Eckhard Thines ¹, and Gero Steinberg ¹^*

¹ Max-Planck-Institut für Terrestrische Mikrobiologie, D-35043 Marburg, Germany

^* To whom correspondence should be addressed. E-mail: gero.steinberg{at}staff.uni-marburg.de.

Fungal chitin synthases (CHSs) form fibers of the cell walland are crucial for substrate invasion and pathogenicity. Filamentousfungi contain up to 10 CHSs, which might reflect redundant functionsor the complex biology of these fungi. Here, we investigatethe complete repertoire of eight CHSs in the dimorphic plantpathogen Ustilago maydis. We demonstrate that all CHSs are expressedin yeast cells and hyphae. Green fluorescent protein (GFP) fusionsto all CHSs localize to septa, whereas Chs5-GFP, Chs6-GFP, Chs7-yellowfluorescent protein (YFP), and Myosin chitin synthase1 (Mcs1)-YFPwere found at growth regions of yeast-like cells and hyphae,indicating that they participate in tip growth. However, onlythe class IV CHS genes chs7 and chs5 are crucial for shapingyeast cells and hyphae ex planta. Although most CHS mutantswere attenuated in plant pathogenicity, ${Delta}$ chs6, ${Delta}$ chs7, and ${Delta}$ mcs1mutants were drastically reduced in virulence. ${Delta}$ mcs1 showed nomorphological defects in hyphae, but Mcs1 became essential duringinvasion of the plant epidermis. ${Delta}$ mcs1 hyphae entered the plantbut immediately lost growth polarity and formed large aggregatesof spherical cells. Our data show that the polar class IV CHSsare essential for morphogenesis ex planta, whereas the classV myosin-CHS is essential during plant infection.